1. Field of the Invention
The present invention relates generally to an acceleration sensor utilizing a piezoelectric element, and more particularly, to an acceleration sensor suitably integrated into an air bag system which is mounted on an automobile, for example.
2. Description of the Background Art
An air bag system which is carried on an automobile is driven in response to acceleration applied upon a collision or the like. In order to ensure the operation of such an air bag system, an acceleration sensor is generally integrated therein. As this type of acceleration sensor, there has been proposed an acceleration sensor employing a piezoelectric element which is deformed in response to acceleration applied thereto to output an electric signal, as disclosed in U.S. Pat. No. 4,700,973, for example.
An example of a well-known conventional acceleration sensor employing a piezoelectric element will be described with reference to FIGS. 8 and 9.
This acceleration sensor has a piezoelectric element 21 and a hybrid IC 22 including a circuit for processing a detection signal which is outputted from the piezoelectric element 21. An end of the piezoelectric element 21 is fixed to a support member 25 which is fixed on a metal base plate 23. A free end of the piezoelectric element 21 is deflected by acceleration applied along a direction G which is orthogonal to the surface of the support member 25 supporting the piezoelectric element 21, to output a detection signal corresponding to the degree of the acceleration.
The hybrid IC 22 is fixed on the metal base plate 23. In addition, the piezoelectric element 21 and the hybrid IC 22 are electrically connected to each other through lead wires 26. Further, a plurality of pin terminals 27 for electrically connecting the acceleration sensor to an external unit or circuit are mounted so as to extend downward from the metal base plate 23. The piezoelectric element 21 and the hybrid IC 22 are sealed with a cap 24 which is fixed to the metal base plate 23.
In the above-described conventional acceleration sensor, the piezoelectric element 21 is supported by the support member 25 in a cantilevered manner, so that the piezoelectric element 21 is bent/deformed in response to the acceleration applied thereto. When extremely large acceleration is applied, therefore, the free end portion of the piezoelectric element 21 swings remarkably. Consequently, the piezoelectric element 21 itself may be broken when exposed to extremely large acceleration or impact, of about 2000 G, for example.
The piezoelectric element 21 and the hybrid IC 22 are connected to each other by a plurality of lead wires 26 in the above-described acceleration sensor. As a result, the lead wires 26 may be disconnected due to metal fatigue caused by vibration. In order to prevent such disconnection, the diameters of the lead wires 26 may be increased, thereby improving strength. If the diameters of the lead wires 26 are increased, however, the lead wires 26 may resonate with vibration following acceleration, which can exert a bad influence on the acceleration detecting operation of the piezoelectric element 21.